Steam engines and other external combustion engines have been known for many years. They have been used on a variety of vehicles and equipment to perform work. For example, they have been used in steamboats, steam locomotives, to power electrical generators and even in some of the very first automobiles. External combustion engines use a fuel source, such as wood or coal, to generate heat. Instead of burning the fuel to directly generate power, this heat is used to heat a working fluid such as water in a boiler to its boiling point. Once the water becomes vapor, that is, steam, additional heat allows the pressure in the boiler to increase. The energy in this pressurized steam is employed to cause the engine to produce power.
For example, in a reciprocating-piston engine, the pressurized steam is supplied from the boiler to the cylinders to cause the pistons to move. The movement of the pistons transfers the energy in the steam to the engine, transforming it into power delivered to a rotating vehicle drive shaft or other device to do work. The steam in the cylinder cools as it expands in the cylinder as the piston moves, increasing the volume of the cylinder. In a double- or triple-expansion engine, the steam is supplied from the exhaust of the first cylinder to a second cylinder, usually of larger capacity, so as to extract further work from further expansion, and so on. The cooled steam is either exhausted by the engine into the atmosphere or condensed for later reheating and resupply to the steam engine.
There are two problems commonly associated with steam engines that make their use in vehicles undesirable, especially in on-demand vehicles such as personal automobiles. First, typical boilers require a significant amount of time to warm up and produce useful quantities of steam. It can take upwards of 5-10 minutes to generate enough steam to move the vehicle at highway speeds. While this amount of time to warm up the boiler is sometimes acceptable in larger, scheduled vehicles, such as trains and boats, it is generally not acceptable in automobiles. Second, typical steam engines require a large-volume boiler for storing the steam as it is generated, prior to supplying the steam to the engine. This large storage area takes up a considerable amount of space in a vehicle that would desirably be available for cargo or passengers. Some improvements were made by the use of faster-heating “flash” boilers, which did not store a large volume of steam, for example as shown in Doble U.S. Pat. No. 1,675,600, but there remain delays and complexities of control that would be unacceptable to today's drivers, who expect to get in the vehicle and drive off without having to consider the operation of the vehicle power plant.